It is well known in automotive engine controls to regulate the released (typically referred to as closed) position of the throttle in the throttle bore of the engine such as by controlling the position of a movable throttle stop in order to achieve a desired engine operating condition. The most common function of such regulation is the closed loop control of engine idle speed. When controlling the engine idle speed, the throttle position and therefore air intake quantity is actively regulated in response to measured engine speed to maintain a scheduled engine idle speed.
The need for a controlled transition to the idle speed control mode when the vehicle operator releases the throttle has long been recognized. For example, to prevent the engine speed from undershooting the idle speed, thereby giving rise to potential engine stalling, or to prevent the increase in hydrocarbon emissions resulting from a deficiency of air, it has been suggested that the released throttle position be established at some controlled transitional throttle angle.
The U.S. Pat. No. 4,848,189 to Simon, Jr. et al discloses such a control system which is directed to an additional function for controlling the throttle position when the throttle is released by the vehicle operator so as to provide for smooth transmission upshifts and a smooth transition to the engine coastdown operation.
To execute the control schemes requiring adjustment of the throttle stop, an electrical motor including a gearset, called herein an idle speed control (ISC) motor, is used to position the stop. Electrical pulses actuate the motor to retract or extend the stop and thereby set the minimum throttle angle. If the motor has a predictable response to the actuating pulses, the motor can be displaced a desired amount according to the pulse width of the actuating pulse to obtain accurate control. Due to manufacturing tolerances, the ISC motors do not all have the same characteristics and thus each vehicle will respond differently to identical control inputs. This condition forces the design to include tradeoffs of response time versus overshoot and undershoot. It is desired therefore to eliminate the system variability due to motor to motor variations so that a consistent system response can be expected.
Another throttle control scheme which requires a motor with a predictable response is an electronic throttle control (also called a drive by wire system) wherein there is no mechanical connection between the accelerator pedal and the throttle. Rather, the throttle blade is positioned solely by a motor which is controlled by the control unit to carry out the throttle response to accelerator pedal input as well as to implement the idle speed control. It is likewise desired for the electronic throttle control system to eliminate system variability due to motor to motor variations so that a consistent system response can be expected.